U.S. Pat. No. 3,889,350, commonly assigned to the assignee herein, outlines a favorable composition of a prealloyed powder metal which is useful in providing excellent hot formed steel when applied to the making of heavily stressed automotive components such as connecting rods, converter lockup clutch races, differential gears and similar parts. This powder metallurgy steel is characterized by a high impact strength of about 40-50 ft. lbs., at 0.35% C, quenched and drawn to R.sub.c 20 (110 KSI UTS, 100 KSI Yield Strength, 28% Elongation, 55% Reduction of Area). The patent taught precise control of alloying ingredients within narrow ranges to allow for maintaining the oxygen content of said powder supply at a low level when subjected to water atomization. It was found that if the principal alloying ingredients, like nickel and molybednum, were controlled to an amount essentially about 0.5% of the mass of powder and manganese controlled to the range 0.3-0.4%, the oxygen could be kept below 0.25%. Unfortunately, such prealloyed powder gives a hardenability slightly less than the now popular modified 4600 powder metallurgy steel composition containing approximately 2% nickel and 0.5% molybdenum with the balance of iron. Thus, even though a successful and less expensive prealloyed powder was formulated, such powder when subjected to a complete powder metallurgy sequence, including hot forming, did not give the type of response to heat treatment that was competitive or advantageous over that currently known. To be successful, powder metallurgy techniques must be able to provide a substitutable product for the same type of steel which is wrought.
A physical multiplying factor must be found through chemistry or process sequence which dramatically improves the hardenability response in powder metallurgy techniques. More specifically, it is most desirable for the prior art to be able to obtain at least a 1.5 inches value for D.sub.i when the powder content contains carbon at about 0.2%, which is a carburizing grade steel.
The prior art has shown an increase in hardenability when prealloyed. However, such improvement in hardenability response is limited to the higher carbon additions. Such steels, however, are not suitable for carburizing as the core toughness decreases with increasing carbon content.